There are many devices which require AC/DC conversion in order to be powered by the mains.
This invention is of particular interest to LED lighting, in which an AC/DC converter supplies an LED driver. The LED driver for example comprises a resonant DC/DC converter, which can be configured or operated as a constant current source or a constant voltage source. A constant current source can be used to drive an LED arrangement directly, thus enabling a single stage driver. Constant voltage sources can be used, for example, for LED modules which have further driver electronics in order to ensure a corresponding power supply to the LEDs with a predetermined current from the output voltage provided by the constant voltage source.
One function implemented within a power converter which is supplied with mains (or other AC) power is power factor correction (PFC). The power factor of an AC electrical power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit. A power factor of less than one means that the voltage and current waveforms are not in phase or distorted, reducing the instantaneous product of the two waveforms. The real power is the capacity of the circuit for performing work in a particular time. The apparent power is the product of the current and voltage of the circuit.
Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power.
If a power supply is operating at a low power factor, a load will draw more current for the same amount of useful power transferred than for a higher power factor.
The power factor can be increased using power factor correction. For linear loads, this may involve the use of a passive network of capacitors or inductors. Non-linear loads typically require active power factor correction to counteract the distortion and raise the power factor. The (passive) power factor correction brings the power factor of the AC power circuit closer to unity by supplying reactive power of opposite sign, adding capacitors or inductors that act to cancel the inductive or capacitive effects of the load.
Active PFC makes use of power electronics to change the waveform of the current drawn by a load to improve the power factor. Active PFC circuits may for example be based on buck, boost or buck-boost switched mode converter topologies. Active power factor correction can be single-stage or multi-stage.
The power factor is typically required to be greater than 0.9 for high power lighting, above 25 W. For professional lighting applications the power factor is also usually required to be equal to or larger than 0.9 even below an input power of 25 W. Since the lighting industry has always been extremely cost driven, the relevant standards (IEC61000-3-2) allow for lower power factors (>0.5) for input powers lower than 25 W. There is a so-called “special waveform” requirement set by the standard that guarantees that although the power factor is lower than 0.9, the mains input current harmonics are still acceptable.
This special waveform was defined such that a typical passive AC-DC converter could be used for lighting, namely the so-called bridge/electrolytic capacitor combination, as shown in FIG. 1.
This comprises a full bridge rectifier 10 receiving a mains input 12 and the rectified DC signal provided by the rectifier is directly connected to a high voltage electrolytic capacitor 14, usually in combination with an AC series resistor for limiting the capacitive inrush current. The output load is represented by resistor RL, and represents any kind of circuitry consuming the input power, e.g. a LED driver. Current is drawn from the mains through an input resistor Rin.
In FIG. 2 the typical waveforms of such a circuit are depicted. Plot 20 shows the mains input voltage (between live and neutral), plot 22 shows the current drawn from the mains through the input resistor Rin and plot 24 shows the output voltage across the capacitor 14 and load RL.
The limitation of this circuit is that the power factor only reaches 0.5 to 0.6, and is therefore not suited to meet higher power factor requirements, for example as required in California, and/or achieve the Energy Star label in the US. Power factor correction circuits achieving power factor of 0.9 are more complex. For example, in some solutions, a power factor boost correcting converter is inserted between the bridge rectifier and the mains storage capacitor. The boost converter enforces a current that is always in phase with and at the same frequency as the line voltage. The resulting power fluctuation at twice the mains frequency is filtered by a high voltage bulk capacitor, limiting the resulting (inherent) output voltage ripple. Another switched-mode converter inside the power supply produces the desired output voltage or current from the DC bus. This example results in a dual stage converter, the first stage is the PFC boost stage and the second stage is the other switched-mode converter to provide the output voltage or current. The bulk capacitor provides the required energy storage to bridge the input power gap when the AC voltage is (close to) zero.
A solution is therefore needed that provides a somewhat higher power factor, for example of at least 0.7, at reasonable cost, without needing to use the more expensive solutions for power factors above 0.9.
US 2008/0123379 discloses a voltage regulating circuit comprising a rectifier for receiving an AC voltage and for generating a rectified AC voltage, and a capacitor connected in parallel with said rectified AC voltage for providing a DC voltage over a load, characterized by a unidirectional current switch provided between the rectifier and the capacitor, and a control block arranged to activate the switch at selected instances during negative slopes of the rectified AC voltage so that said DC voltage does not exceed a predetermined voltage limit. By controlling the voltage provided by the rectified mains, the DC voltage can be regulated to any preset value (lower than the AC mains peak value). The inventive voltage stabilizer will guarantee a desired constant DC load voltage value for different mains peak input voltages and under wide range of load variations. Thereby a converter driven by this voltage can be more optimized or even be unregulated.
US 2013/0049618 discloses an adaptive circuit for driving a lower-voltage DC load from a rectified higher-voltage AC supply, which adaptive circuit comprises a charge-storage circuit, which charge storage circuit comprises a first capacitor and a second capacitor connected essentially in series, wherein the second capacitor is connected at least in parallel with the load; and an active switch realised as a controlled current source for controlling a load current through the load such that, in a closed switch state, load current is drawn essentially from the first capacitor of the charge-storage circuit, and, during an open switch state, load current is drawn essentially from the second capacitor. The invention also describes an LED retrofit lamp comprising a connecting means for connecting the lamp to a higher-voltage mains supply signal; an LED device rated for a lower-voltage supply; and such an adaptive circuit for adapting the higher-voltage mains supply signal to a low-voltage signal for driving the lower-voltage LED device. The invention also describes a method of driving a lower voltage DC load from a rectified higher-voltage AC supply.